When should i change the oil and do a tune up?

I bought my 2005 Ford Mustang GT in August 2013 used and I don't know when I should do a tune up or an oil change. When I bought the car it had about 135,000 miles on it and it currently has about 140,000 to 150,000 on it so far. I can't tell exact mileage because the gas is screaming at me right now. Serious answers only.

Added (1). My car lets me know when my gas is low and won't stop telling me know till I put more gas in the car. That's what I meant when I said the gas is screaming at me. My car is a 4.6 liter V8

Added (2). The person who owned this car before me never kept records.

Oil and filter every 3K and and look at the plugs to see if they have been changed (dirt residue will usually give it away if they have been changed), The good plugs are serviceable for 100K miles. If you do pull a spark plug for inspection / gap, start with the back 2 first, if they are good the rest are too. You didn't give an engine spec. If it has aluminum heads, you may be better off taking it to a mechanic. Aluminum heads are persnickety on the plugs and you can easily strip one out.

(A) is correct. But i bet there's an owners manual in the glove box that tells you.

It's not so much how often or what's called "Oil Change Interval", it's the quality of the motor lubricant. Notice I use the word, "lubricant" because I emphasize people use quality synthetic, which in my opinion determines the oil change interval in vehicles that are in good running condition. Quality filters are almost just as important as the engine lubricant. Quality air & oil filters are not rated or spec'd like oil is so do some research to find the filter with the best efficiency.

Change the oil every 4 months or 4000 miles.
A tune up might result in better gas mileage.
What do you mean by, "the gas is screaming at me right now"?

Change oil every 3 to4 thousand miles.It may need spark plugs so it will have better go power.